Standing wave type linear accelerators generate high-energy electron beams which strike metallic targets to generate x-rays. The linear accelerators have a series of linearly arranged cavity resonators separated by apertured walls. The apertures define a passage through which the electron beam travels to interact with standing waves supported in the cavities. The beam gains energy as it travels through successive resonant cavities. The electrons are injected into the first cavity at relatively low energy by an electron gun. The electron beam is accelerated as it travels through the cavities. Electrons which strike cavity walls during their travel through the accelerator not only reduce the electron current reaching the x-ray target but also generate undesirable leakage x-ray radiation. The electrons striking the target generate x-rays which are emitted in all directions. Forward traveling x-rays are intercepted by a beam blocker which includes an aperture which defines the shape of the desired beam. The accelerator and the target region are shielded to absorb the leakage x-ray radiation and the target radiation except for the desired radiated beam. The x-ray shielding adds weight and size to the x-ray source.